The present invention provides thermoformable, multi-layered, symmetric, coextruded films containing polyamides, polyolefins or copolymers based on olefins. These films are particularly suitable for extreme thermoforming applications with low moulding temperatures which require very high puncture-resistance. Since the film according to the invention is completely symmetric, it has no tendency to roll up. This is an important criterion for plastics packaging in order to be able to present the packages attractively in cash-and-carry outlets.
To simplify the discussion of the prior art and the invention, the polymers which are used for films in the area of the present invention and the abbreviations used for these polymers for use in the rest of the description are explained in more detail in the following. If specific polymers are mentioned here, then these are suitable for the corresponding layers in the film according to the invention.
Polyamides (abbreviated to xe2x80x9cPAxe2x80x9d) are understood in the widest sense to be polymeric compounds which are linked together by the acid amide group Rxe2x80x94NHxe2x80x94COxe2x80x94Rxe2x80x2. Two groups of polyamides are differentiated: those built up from one parent substance by polycondensation of xcfx89-aminocarboxylic acids or polymerisation of their lactams to give polyamide-6 type compounds and those which are produced from two parent substances, diamines and dicarboxylic acids by polycondensation to give polyamide-66 type compounds. In addition, copolyamides are also known, these being a combination of the types of polyamide mentioned.
Polyolefins are collectively summarised as xe2x80x9cPOxe2x80x9d. Examples of polyolefins are low density polyethylene (LDPE), high density polyethylene (HDPE), linear low density polyethylene (LLDPE), polypropylene (PP), polyisobutylene, polybutylene and all other polymers which are prepared from olefins.
Furthermore, PO here also includes olefin copolymers consisting of olefins and other monomers, wherein the olefins predominate in the composition. These are, for example, ethylene copolymers with unsaturated esters (e.g. vinyl acetate) and ethylene copolymers with unsaturated carboxylic acids or their salts.
xe2x80x9cPPxe2x80x9d is used for polypropylene, no matter what the spatial arrangement of the methyl side-groups. xe2x80x9cEPCxe2x80x9d is used for copolymers of propylene with 1-10 mol. % of ethylene, wherein the ethylene is randomly distributed in the molecule.
xe2x80x9cLDPExe2x80x9d is used for low density polyethylene which has a density in the range 0.86 to 0.93 g/cm3. LDPE molecules are characterised by a high degree of branching.
xe2x80x9cLLDPEsxe2x80x9d are linear low density polyethylenes which contain, apart from ethylene, one or more xcex1-olefins with more than 3 carbon atoms as comonomers. The following are mentioned here as representative of xcex1-olefins: butene-1, hexene-1,4-methylpentene-1 and octene-1. By polymerisation of the substances mentioned, the typical molecular structure of LLDPEs is produced, which is characterised by a linear main chain with side chains attached thereto. The density varies between 0.86 and 0.935 g/cm3. The melt flow index MFI is normally between 0.3 and 8 g/10 min. In some publications, linear ethylene/xcex1-olefin copolymers are subdivided into VLDPEs and ULDPEs, in accordance with the density. Since, however, according to GNAUCK/FRxc3x9cNDT (Einsteig in die Kunststoffechemie Hanser-Verlag 1991, p. 58), the properties, processing and use of these copolymers corresponds largely to those of ethylene homopolymers, the more precise differentiation is not used here.
xe2x80x9cHDPExe2x80x9d is used to describe linear polyethylenes of high density which have only a small degree of branching in the molecular chain. The density of HDPE may be between 0.9 g/cm3 and 0.97 g/cm3.
xe2x80x9cmPExe2x80x9d is used here to describe an ethylene copolymer which was polymerised using metallocene catalysts. An xcex1-olefin with four or more carbon atoms is preferably used as a comonomer. The polymers prepared using conventional Ziegler-Natta catalysts frequently contain higher concentrations of the xcex1-olefins in the lower molecular weight fractions. As a result of the very uniform catalytic effect of metallocene centres, narrow distributions of molecular weights are found and, during fractionation, very uniform incorporation of the xcex1-olefins in both the high and also the low molecular weight fractions. The density is preferably between 0.900 and 0.930 g/cm3. The molecular weight distribution Mw/Mn is less than 3.5, preferably less than 3.
xe2x80x9cEAAxe2x80x9d is used for copolymers consisting of ethylene and acrylic acid and xe2x80x9cEMAAxe2x80x9d for copolymers consisting of ethylene and methacrylic acid. The ethylene content is preferably between 60 and 90 mol. %.
xe2x80x9cIxe2x80x9d is used to describe olefin-based copolymers in which the molecules are cross-linked via ionic bonding. Ionic cross-linking is reversible, which results in breaking up of the ionic cross-linking at conventional processing temperatures (180xc2x0 C. to 290xc2x0 C.) and fresh formation in the cooling phase. Copolymers of ethylene and acrylic acid, which are cross-linked with sodium or zinc ions, are normally used as polymers.
xe2x80x9cEVAxe2x80x9d is used for a copolymer consisting of ethylene and vinyl acetate. The ethylene content is preferably between 60 and 99 mol. %.
xe2x80x9cHVxe2x80x9d is used for coextrudable, adhesion-promoting polymers (bonding agents). In contrast, adhesives arc understood to be non-metallic materials, including polymers, which can bond two layers of film due to surface adhesion and internal strength without substantially altering the structure of the bonded film layers. In contrast to bonding agents, adhesives are not coextrudable, but have to be applied separately by surface application (e.g. lamination) or liquid application. Bonding agents are preferably modified polyolefins such as e.g. LDPE, LLDPE, mPE, EVA, EMAA, PP, EPC which are grafted with at least one monomer from the group of xcex1,xcex2-monounsaturated dicarboxylic acids such as, for example, maleic acid, fumaric acid, itaconic acid or their acid anhydrides, acid esters, acid amides or acid imides. Other extrudable bonding agents which may be used are also copolymers of ethylene with xcex1,xcex2-monounsaturated monocarboxylic acids such as acrylic acid, methacrylic acid and/or their metal salts with zinc or sodium and/or their C1-C4-alkyl esters, which may also be grafted with at least one monomer from the group of xcex1,xcex2-monounsaturated dicarboxylic acids such as, for example, maleic acid, fumaric acid, itaconic acid or their acid anhydrides, acid esters, acid amides or acid imides. In addition, polyolefins such as e.g. polyethylene, polypropylene, ethylene/propylene copolymers or ethylene/xcex1-olefin copolymers which are grafted with copolymers of ethylene with xcex1,xcex2-monounsaturated monocarboxylic acids such as acrylic acid, methacrylic acid and/or their metal salts with zinc or sodium and/or their C1-C4-alkyl esters, may be used. Particularly suitable bonding agents are polyolefins, in particular ethylene/xcex1-olefin copolymers with grafted xcex1,xcex2-monounsaturated dicarboxylic anhydrides, in particular maleic anhydride. HVs may also contain an ethylene/vinyl acetate copolymer, preferably with a vinyl acetate content of at least 10 wt. %.
Different layers in a film are separated in the following by xe2x80x9c/xe2x80x9d. Mixtures of polymers in a layer are signified by round brackets and linked by a xe2x80x9c+xe2x80x9d. Thus the film structure xe2x80x9c . . . /LLDPE/(mPE+LDPE)xe2x80x9d describes a multi-layered film, wherein the outer layer contains a mixture of mPE and LDPE. The inner layer consists of LLDPE. The other layers in the film are indicated by xe2x80x9c . . . xe2x80x9d.
The following documents are regarded as the relevant prior art for the present invention:
DE 3 216 097 C3 describes a process for a two-layered blown film consisting of polyethylene and ethylene copolymers which is prepared by blocking of the inner layer. The inner layer consists of an ethylene/vinyl acetate copolymer or ethylene/methacrylic acid copolymers which are partly cross-linked with ions of sodium or zinc. The ethylene/vinyl acetate copolymer contains up to 30 wt. % of vinyl acetate, preferably 3 to 10 wt. % of vinyl acetate. This film is laminated with a monolayered or multilayered support film and is characterised by its elevated puncture-resistance, planarity and wear properties with regard to metal. EP 0 219 329 B1 describes a multi-layered, coextruded film with a very high oxygen barrier effect. This is a 7-layered blown film which is prepared by blocking the inner layer, consisting of an ethylene/vinyl acetate copolymer with itself. The film does not contain any polyamide, rather EVOH and PVDC as barrier layers.
EP 0 613 774 A1 describes a multi-layered film based on polyamide and olefinic polymers or copolymers which has the main structure PA*/PE*/PA**/PE**. Each of these four layers can in turn consist of several layers of the same type, E.g. PA* may consist of various PA6/PA6/12/PA6s. The same applies to the PE layers; e.g. PE** may consist of two layers of an extrudable bonding agent (HV) and LDPE as a sealing layer.
U.S. Pat. No. 4,909,726 describes a multilayered film consisting of a blown film which contains a an inner layer which can seal to itself consisting of a modified polyethylenic bonding agent. Furthermore, the film contains two polyamide layers, between which is embedded an EVOH layer. The film is preferably produced as a blown tubular film, wherein the inner layers are welded to each other. Films of this type are suitable in particular for vertical moulding and filling sealing machines in which the fill is shaped to give a tube. The longitudinal seam is prepared with heat-sealing bars, the ends of the tube are clipped. The film described exhibits very high impact-resistance. Filled packages are allowed to fall from various heights and the number of packages which break open is recorded. This type of impact strain is not comparable with the completely different type of strain due to pointed or sharp-edged items which puncture the film.
All the films in accordance with the prior art exhibit the common features that they are not sufficiently thermoformable, they do not have a low tendency to roll and they are not puncture-proof.
Thus, there is the object of providing a film with improved
puncture resistance
thermoformability and
a low tendency to roll.